1. Field of the Invention
The present invention relates to a wireless communication system that changes an access point (base station) to which a wireless communication terminal such as an internet protocol (IP) telephone terminal is connected when the wireless communication terminal moves between different networks while communicating with a communication party.
2. Related Art
In recent years, a wireless communication system has changed to enable an IP telephone terminal connected with an access point among a plurality of access points through a wireless local area network (LAN) or the like to continue to communicate with a communication destination terminal by enabling the IP telephone to perform the handover of the access point to the one having a higher signal transmission power level when the IP telephone moves between different subnetworks, owing to the development of an IP network communication technology and a mobile communication technology (see, for example, JP-2005-6216A).
Now, the handover processing will be described with reference to FIG. 21.
FIG. 21 shows an IP telephone system 500 in which an IP telephone terminal 502 and a call destination terminal 503 are connected with each other through a predetermined network circuit N to perform packet communication (calls). For example, when a user carrying the IP telephone terminal 502 moves from a position A to a position C passing through a position B, then the received power levels of the signals from a first access point 501A become gradually lower.
In this case, when the IP telephone terminal 502 detects the lowering of the received power levels, the IP telephone terminal 502 performs processing of changing access points 501.
Concretely, first, the IP telephone terminal 502 transmits a probe request and detects the received power levels of the probe responses transmitted from the neighbor access points 501 to the probe request. Then, the IP telephone terminal 502 sets the access point which sent the probe responses with the highest level (for example, which is the access point 501B in FIG. 21) as the new access point 501 of the handover destination. When the new access point 501 of the handover destination is determined, the IP telephone terminal 502 establishes a wireless link with the access point 501B.
Next, the IP telephone terminal 502 accesses a dynamic host configuration protocol (DHCP) server (not shown) to obtain a new IP address. For example, the IP address thereby comes to be changed from “192.168.1.10” to “192.168.2.10.” The IP telephone terminal 502 successively transmits a request to notify the call destination terminal 503 of the change of the own IP address of the IP telephone terminal 502, to a session initiation protocol (SIP) server 506.
When the SIP server 506 receives the request from the IP telephone terminal 502, the SIP server 506 transmits the request to the call destination terminal 503.
When the call destination terminal 503 receives the request from the SIP server 506, the call destination terminal 503 performs the processing of dealing with the change of the IP address of the IP telephone terminal 502. Now the handover processing is set to be performed when an IP telephone terminal detects the lowering of the received power levels and judges to change the access point. In this case, a call between IP telephony terminal and the call destination terminal is interrupted until a SIP server completes registration of new IP address, which is obtained by the IP telephony terminal after access point change. If such an interruption continues several seconds, the commercial value of the IP telephone will decrease.
Meanwhile, as a technology for continuing communication with a communication destination terminal even when a terminal moves between different subnetworks while holding the IP address thereof, for example, the “mobile IP” technology and the like, which are defined by Request for Comment (RFC) 2002 and RFC 3220 of Internet Engineering Task Force (IETF), are also known.
However, because the mobile IP system is is not cost effective due to its large scale. Furthermore, the system is configured to perform the transmission and the reception of information through a fixed “home agent”. Due to this, increase of the distance from an IP telephone terminal to the “home agent” leads to increase of intermediate networks. The increase of intermediate networks cause wasteful data traffic and decreases efficiency of whole network.